This invention relates to a tubular incandescent lamp and more particularly to a tubular incandescent lamp favorably applied to an electronic copying machine for ordinary paper.
With a copying machine based on the principle of electrophotography or xerography, a static latent image is formed on the photoelectric surface of a transcription selenium drum. After the image is covered with toner powder, the latent image is developed. The developed image is then transcribed on ordinary paper. After completion of transcription, a cleaning step is taken which irradiates light beams on the photoelectric surface of the selenium drum for discharge in order to remove toner powder still remaining on the surface. A fluorescent lamp is generally used as a source of light for this cleaning step. However, the fluorescent lamp requires a stabilizer and is expensive. Moreover, the circuit of a microcomputer installed in the recent type of copying machine is harmfully affected by pulses issued when the fluorescent lamp is lighted. For this reason, an incandescent lamp has come to be used as a source of light for the removal of toner powder remaining on the photoelectric surface of the transcription selenium drum. An incandescent lamp used for this particular purpose includes the type formed by placing one long lengthwise extending helically coiled filament in a narrow tubular bulb, and the type constructed by linearly arranging a plurality of short helically coiled filaments in a narrow bulb. However, these types of incandescent lamp have the drawbacks that they have low resistance to shocks and vibrations, release a great deal of heat to increase the temperature of the photoelectric surface of the transcription selenium drum, possibly leading to a decline in the performance of said selenium drum. Accordingly, the present inventors previously proposed a tubular incandescent lamp. This proposed tubular incandescent lamp comprised a plurality of small lamps linearly arranged and extending lengthwise through a narrow transparent tubular envelope. This proposed incandescent lamp had the advantages that it had high resistance to shocks and vibrations, and released a relatively small amount of heat, ensuring a small rise in the temperature of the photoelectric surface of the transcription selenium drum. Later, however, it was found that while the proposed incandescent lamp was used, the angles which light beams emitted from the plural small lamps defined with the lateral walls of the envelope of the incandescent lamp were displaced from the prescribed direction, thereby resulting in wide variations in the intensity of light beams projected from the small lamp to the photoelectric surface of the transcription selenium drum. Such undesirable event should be avoided.